Due to their many advantages, plastics have become inescapable in the mass production of objects. Aromatic polyesters, such as polyethyleneterephthalate (PET), which is a polyester comprising ethylene glycol and terephthalic acid units, are used, for example, for the manufacture of containers, packagings or textile fibres.
The term “monomer units” is understood to mean, according to the invention, units included in the polyester, which units can be obtained after polymerization of a monomer. As regards the ethylene glycol and terephthalic acid units included in PET, they can either be obtained by an esterification reaction of ethylene glycol and terephthalic acid or by a transesterification reaction of ethylene glycol and terephthalic acid ester.
The development of polyesters resulting from biological resources renewable in the short term has become an ecological and economic imperative, in the face of the exhaustion and of the increase in costs of fossil resources, such as oil. One of the main concerns today in the field of polyesters is thus that of providing polyesters of natural origin (biosourced polyesters). Thus, groups such as Danone or Coca-Cola are today marketing drink bottles made of partially biosourced PET, this PET being manufactured from biosourced ethylene glycol. A disadvantage of this PET is that it is only partially biosourced since the terephthalic acid, for its part, results from fossil resources. Although polyesters comprising biosourced terephthalic acid have already been described, for example in Application WO 2013/034743 A1, the processes for the synthesis of biosourced terephthalic acid or biosourced terephthalic acid ester remain too expensive to date for completely biosourced PET to currently experience commercial success.
Other aromatic polyesters, comprising monomer units other than terephthalic acid units, have been manufactured in order to replace PET.
Among biosourced polyesters, aromatic polyesters comprising 2,5-furandicarboxylate units constitute an advantageous alternative as these polyesters exhibit mechanical, optical and thermal properties similar to those of PET.
These polyesters have been described in various documents. Patent Application US 2011/0282020 A1 describes in particular a process for the manufacture of a polyester comprising 2,5-furandicarboxylate units in which:                in a first stage, a 2,5-furandicarboxylic acid ester is reacted with a polyol in the presence of a transesterification catalyst comprising Sn(IV), in order to form a prepolymer;        then, at reduced pressure, in a second stage, the prepolymer thus formed is polymerized in the presence of a polycondensation catalyst comprising Sn(II) in order to increase the molar mass thereof and to form a polyester.        
This process makes it possible to manufacture polyesters of high molecular weight and low coloration, without requiring a stage of purification after synthesis.
However, the Applicants were able to find that, for some applications or under some conditions of use, these polyesters did not exhibit all the required properties. This is, for example, the case for applications requiring that the polyester exhibit a high glass transition temperature. By way of example, when it is desired to fill, under hot conditions, bottles formed of polyester, it is desirable for the glass transition temperature to be as high as possible, in order for the bottle to retain its shape during the filling. Other examples of applications in which it is necessary to have such polyesters having a high glass transition temperature are the articles intended to be placed close to heat sources, as is the case, for example, with headlamps or bulbs which emit heat when they are used. This is because it is necessary to retain the dimensional stability of these objects over time; in point of fact, the higher the glass transition temperature, the better the dimensional stability in the event of exposure of the article to heat.
The document US 20130095269 A1 for its part describes, in a general manner, copolyesters comprising 2,5-furandicarboxylate and ethylene glycol units and at least one additional glycol. The use of additional glycol does not make it possible to increase the glass transition temperature. This is because, contrary to polyesters of PET type, where the glass transition temperature is increased (see Examples 4 and 5), the use of an additional glycol in polyesters comprising 2,5-furandicarboxylate and saturated diol units reduces their glass transition temperature (Examples 1 and 2). These polyesters are obtained by a synthetic process using, as monomer, 2,5-furandicarboxylic acid (FDCA).
The document US 20130171397 A1 describes the manufacture of polyesters comprising 2,5-furandicarboxylate units. Among these various polyesters, polyesters of 2,5-furandicarboxylic acid and ethylene glycol (PEF) and also copolyesters of 2,5-furandicarboxylic acid and a mixture of glycols consisting of ethylene glycol and bicyclic diol—more particularly isosorbide—(PEIF) are synthesized. The introduction of a bicyclic diol does not make it possible to increase the glass transition temperature since that of PEF is 79° C. while that of the polyesters comprising bicyclic diol units is at the very most 78° C. (see Table 4). These polyesters are obtained by a synthetic process using, as monomer, 2,5-furandicarboxylic acid.
It can thus be concluded, from the teachings of these two documents, that the use of glycols to modify a polyester comprising 2,5-furandicarboxylate and saturated diol units, in particular the use of a bicyclic diol, does not make it possible to increase the glass transition temperature of this polyester.
There thus still exists a need to obtain novel polyesters which can be partially or completely biosourced and which exhibit an improved glass transition temperature.
In the context of their research studies, the Applicants have succeeded in improving the thermal properties of polyesters comprising 2,5-furandicarboxylate and saturated diol units, the said saturated diol being linear or branched, and in thus obtaining a polyester having a higher glass transition temperature. Unexpectedly and contrary to what is taught in the prior art, they have succeeded in obtaining this novel polyester by using a bicyclic diol, starting from a specific process described in the continuation of the description.